define_expand

The define_insn definition describes an insn implementing the named functionality (used when converting the higher level IR to RTL), but there are cases where the named functionality must expand to more than one insn, or when the RTL should be generated differently depending on the operands – this is handled by define_expand.

The general format of define_expand is essentially the same as for define_insn – it consists of a name, an RTL template, a condition string containing C++ code to enable/disable the instruction pattern, and a section of C++ code (called preparation statements) that can be used to generate RTL.

Our first example of how to use define_expand is from the Arm handling of rotlsi3 (“rotate left”) – Arm does only have a “rotate right” instruction, but rotating left by x is the same as rotating right by 32-x, and we can generate this as:

The preparation statements are used to modify the second operand given to rotlsi3 to calculate the correct value for the code generated by the RTL template.

define_expand may choose to skip the generation of the RTL template by executing DONE in the preparation statements. An example of how this is used is the Moxie implementation of mulsidi3 that creates a 64-bit result from multiplying two 32-bit values. The Moxie back end generates this pattern as two instructions (one that calculates the upper 32 bits, and one that calculates the lower 32 bits) where all work is done by the preparation statements

DONE works as a return statement, and it can be used in conditional code to let the preparation statements override the RTL template for special cases while using the RTL template for the general case. The Arm smaxsi3 instruction pattern (calculating the maximum of two signed integers) uses this to handle two special cases more efficiently:

The problem it solves is that the general case translates to the instructions

cmp r1, r2
movge r0, r2
movlt r0, r1

which clobbers the condition code, but the special cases max(x,0) and max(x,-1) can be generated as

bic r0, r1, r1, asr #31

and

orr r0, r1, r1, asr #31

which do not clobber CC. This optimization could be handled by later optimization passes, but doing it as early as possible (i.e. when generating the RTL) gives more freedom to all later passes to optimize cases that otherwise would have been prevented by the clobbering.

Note: The RTL always have the constant as the second operand for commutative binary operations, so the code does not need to check the first operand.

One other way to return from the preparation statements is to execute FAIL which has the effect of ignoring the instruction pattern in the same way as if the pattern had been disabled by the condition string. An example of this is the AArch64 movmemdi instruction pattern implementing a memory block move:

This calls the target-specific aarch64_expand_movmem function that checks if it makes sense to expand the block move inline (that is, if the result will be relatively small) and generates a sequence of move insns if that is the case. If not, it just returns false, which makes this pattern call FAIL, and GCC will ignore this instruction pattern and generate a call to memcpy instead.

The unspec and unspec_volatile expression codes

The RTL template in define_insn contains expressions describing the functionality of the instruction pattern, which enables the optimizers to reason about the insn. Many architectures have instructions that cannot be described by this (or where it does not make sense to describe the functionality, such as instructions for AES encryption – the optimizers cannot take advantage of this description anyway). These cases are handled by describing the instructions using an unspec or unspec_volatile expression which the compiler treats as a black box – the only knowledge the compiler has is what is described by the predicates and register constraints.

One example of how this is used the AArch64 set_fpcr insn that writes to the floating-point control register

This describes a volatile instruction (i.e. an instruction with side effects) that takes a register operand. The last operand to the unspec and unspec_volatile expressions is an integer that identifies the instruction (the backend may have several different unspec instructions, and each gets a different number) – these are by convention defined as enumerations called unspec and unspecv

Attributes

It is possible to add extra information to an insn (such as the length or scheduling constraints) that the back end may take advantage of when generating the code. This is done by defining attributes

(define_attr namelist-of-valuesdefault)

where

name is a string containing the name of the attribute.

list-of-values is either a string that specifies a comma-separated list of values that can be assigned to the attribute, or an empty string to specify that the attribute takes numeric values.

default is an expression that gives the value of this attribute for insns whose definition does not include an explicit value for the attribute.

For example,

(define_attr "length" "" (const_int 2))

defines a numeric attribute “length” having the default value 2. The back end may define attributes with any name, but a few names have a specific usage in GCC. For example, the length attribute contains the instruction’s length measuerd in bytes, and is used when calculating branch distance for architectures where different instruction are used for “short” and “long” branches.

Attribute values are assigned to insns by attaching a set_attr to the define_insn as in

_Base_bitset has a constexpr constructor, and this makes GCC end up building the whole elems array in memory at compile time. The array is large (1,250,400,000 bytes) and GCC need to use even more memory as it represents the array by building AST nodes for the elements.

The constexpr keyword does not mean that the compiler must evaluate at compile time – it only means that it can be evaluated at compile time if the result is used where only constant expressions are allowed. I had assumed that compile-time evaluation is not needed in this case, but GCC seems to always evaluate constexpr at compile time when instantiating templates. Anyway, GCC could use a more efficient representation that does not need to keep the whole array expanded in memory...

The C-style array is not expanded in memory at compile time as it is not defined as a template. The bitset class is still expanded, but it is small, and the compiler only wastes 12,504 bytes of memory by expanding it.

Saturday, September 16, 2017

GCC can warn about questionable constructs in the source code, but most such warnings are not enabled by default – developers need to use the options -Wall and -Wextra to get all generally useful warnings. There are many additional warning options that are not enabled by -Wall-Wextra as they may produce too many false positive warnings or be targeted to a specific obscure use case, but I think a few of them (listed below) may be useful for general use.

Note: -Wjump-misses-init was added in GCC 4.5.Note: -Wjump-misses-init is only available for C – jumping over variable initialization is an error in C++.

-Wdouble-promotion

Warn when a value of type float is implicitly promoted to double.

Floating point constants have the type double, which makes it easy to accidentally compute in a higher precision than intended. For example,

floatarea(float radius)
{
return3.14159 * radius * radius;
}

does all the computation in double precision instead of float. There is normally no difference in performance between float and double for scalar x86 code (although there may be a big difference for small, embedded, CPUs), but double may be much slower after vectorization as only half the number of elements fit in the vectors compared to float values.

Note: -Wdouble-promotion was added in GCC 4.6.

-Wshadow

Warn when a local variable or type declaration shadows another variable, parameter, type, or class member.

-Wformat=2

The -Wformat option warns when calls to printf, scanf, and similar functions have an incorrect format string or when the arguments do not have the correct type for the format string. The option is enabled by -Wall, but it can be made more aggressive by adding -Wformat=2 which adds security-related warnings. For example, it warns for

#include <stdio.h>voidfoo(char *p)
{
printf(p);
}

that may be a security hole if the format string came from untrusted input and contains ‘%n’.

where the compiler will find three possible values for Do: EraseAll, LsAll, and 0.

The value 0 is eliminated from the set of possible values for the call in main, in the same way as for the simpler case, but the compiler cannot change the indirect call to a direct call as there are still two possible values for the function pointer, and clang generates the expected

main:
jmpq *Do(%rip)

But a compiler could transform the line

return Do();

to

if (Do == LsAll)
return LsAll();
elsereturn EraseAll();

that has the same surprising effect of calling a never-called function. This transformation would be silly in this case as the cost of the extra comparison is similar to the cost of the eliminated indirect call, but it may be a good optimization when the compiler can determine that the result will be faster (for example, if the functions can be simplified after inlining). I don’t know if this is implemented in clang/LLVM — I could not get this to happen when writing some small test-programs. But, for example, GCC’s implementation of devirtualization can do it if -fdevirtualize-speculatively is enabled, so this is not a hypothetical optimization (GCC does, however, not take advantage of undefined behavior in this case, so it will not insert calls to never-called functions).

That is, the compiled program executes “rm -rf /” even though the original program never calls EraseAll!

Clang is allowed to do this – the function pointer Do is initialized to 0 as it is a static variable, and calling 0 invokes undefined behavior – but it may seem strange that the compiler chooses to generate this code. It does, however, follow naturally from how compilers analyze programs...

Eliminating function pointers can give big performance improvements – especially for C++ as virtual functions are generated as function pointers and changing these to direct calls enable optimizations such as inlining. It is in general hard to track the possible pointer values through the code, but it is easy in this program – Do is static and its address is not taken, so the compiler can trivially see all writes to it and determines that Do must have either the value 0 or the value EraseAll (as NeverCalled may have been called from, for example, a global constructor in another file before main is run). The compiler can remove 0 from the set of possible values when processing the call to Do as it would invoke undefined behavior, so the only possible value is EraseAll and the compiler changes

return Do();

to

return EraseAll();

I’m not too happy with taking advantage of undefined behavior in order to eliminate possible pointer values as this has a tendency to affect unrelated code, but there may be good reasons for clang/LLVM doing this (for example, it may be common that devirtualization is prevented as the set of possible pointer values contain a 0 because the compiler finds a spurious pure virtual function).

Update: I wrote a follow-up post discussing a slightly more complex case.

Tuesday, August 29, 2017

The functionality that cannot be handled by the machine description in machine.md is implemented using target macros and functions in machine.h and machine.c. Starting from an existing back end that is similar to you target will give you a reasonable implementation for most of these, but you will need to update the implementation related to the register file and addressing modes to correctly describe your architecture. This blog post describes the relevant macros and functions, with examples from the Moxie back end.

Inspecting the RTL

Some of the macros and functions are given RTL expressions that they need to inspect in order to do what is expected. RTL expressions are represented by a type rtx that is accessed using macros. Assume we have a variable

rtx x;

containing the expression

(plus:SI (reg:SI 100) (const_int 42))

We can now access the different parts of it as

GET_CODE(x) returns the operation PLUS

GET_MODE(x) returns the operation’s machine mode SImode

XEXP(x,0) returns the first operand – an rtx corresponding to (reg:SI 100)

XEXP(x,1) returns the second operand – an rtx corresponding to (const_int 42)

XEXP() returns an rtx which is not right for accessing the value of a leaf expression (such as reg) – those need to be accessed using a type-specific macro. For example, the integer value from (const_int 42) is accessed using INTVAL(), and the register number from (reg:SI 100) is accessed using REGNO().

For an example of how this is used in the back end, suppose our machine have load and store instructions that accept an address that is a constant, a register, or a register plus a constant in the range [-32768, 32767], and we need a function that given an address expression tells if it is a valid address expression for the target. We could write that function as

The names are used for generating the assembly files, and for the GCC extension letting programmers place variables in specified registers

registerint *foo asm ("$r12");

The Moxie architecture does only have 18 registers, but it adds two fake registers ?fp and ?ap for the frame pointer and argument pointer (used to access the function’s argument lists). This is a common strategy in GCC back ends to simplify code generation and elimination of unneeded frame pointer and argument pointers, and there there is a mechanism (ELIMINABLE_REGS) that rewrites these fake registers to real registers (such as the stack pointer).

The GCC RTL contains virtual registers (which are called pseudo registers in GCC) before the real registers (called hard registers) are allocated. The registers are represented as an integer, where 0 is the first hard register, 1 is the second hard register, etc., and the pseudo registers follows after the last hard register. The back end specifies the number of hard registers by specifying the first pseudo register

#define FIRST_PSEUDO_REGISTER 20

Some of the registers, such as the program counter $pc, cannot be used by the register allocator, so we need to specify which registers the register allocator must avoid

It is possible to specify the order in which the register allocator allocates the registers. The Moxie back end does not do this, but it could have done it with something like the code below that would use register 2 ($r0) for the first allocated register, register 3 ($r1) for the second allocated register, etc.

For an example of where this can be used, consider an architecture with 16 registers and “push multiple” and “pop multiple” instructions that push/pop the last \(n\) registers. The instructions are designed to store call-saved registers when calling a function, and it makes sense to allocate the call-saved registers in reverse order so the push/pop instructions save as few registers as possible

There are usually restrictions on how registers can be used (e.g. the Moxie $pc register cannot be used in arithmetic instructions), and these restrictions are described by register classes. Each register class defines a set of registers that can be used in the same way (for example, that can be used in arithmetic instructions). There are three standard register classes that must be defined

ALL_REGS – containing all of the registers.

NO_REGS – containing no registers.

GENERAL_REGS – used for the ‘r’ and ‘g’ constraints.

and the back end can add its own as needed. The register classes are implemented as

REG_CLASS_CONTENTS consists of a bit mask for each register class, telling with registers are in the set. Architectures having more than 32 registers use several values for each register class, such as

{ 0xFFFFFFFF, 0x000FFFFF } /* All registers */ \

The back end also need to implement the REGNO_REG_CLASS macro that returns the register class for a register. There are in general several register classes containing the registers, and the macro should return a minimal class (i.e. one for which no smaller class contains the register)

The back end may also need to set a few target macros detailing how values fit in registers, such as

/* A C expression that is nonzero if it is permissible to store a value of mode MODE in hard register number REGNO (or in several registers starting with that one). */ #define HARD_REGNO_MODE_OK(REGNO,MODE) 1

Addressing modes

Addressing modes varies a bit between architectures – most can do “base + index” addressing, but there are often restrictions on the format of the index etc. The general functionality is described by five macros:

/* Specify the machine mode that pointers have. After generation of rtl, the compiler makes no further distinction between pointers and any other objects of this machine mode. */#define Pmode SImode/* Maximum number of registers that can appear in a valid memory address. */#define MAX_REGS_PER_ADDRESS 1/* A macro whose definition is the name of the class to which a valid base register must belong. A base register is one used in an address which is the register value plus a displacement. */#define BASE_REG_CLASS GENERAL_REGS/* A macro whose definition is the name of the class to which a valid index register must belong. An index register is one used in an address where its value is either multiplied by a scale factor or added to another register (as well as added to a displacement). */#define INDEX_REG_CLASS NO_REGS/* A C expression which is nonzero if register number NUM is suitable for use as a base register in operand addresses. */#ifdef REG_OK_STRICT#define REGNO_OK_FOR_BASE_P(NUM) \ (HARD_REGNO_OK_FOR_BASE_P(NUM) \ || HARD_REGNO_OK_FOR_BASE_P(reg_renumber[(NUM)]))#else#define REGNO_OK_FOR_BASE_P(NUM) \ ((NUM) >= FIRST_PSEUDO_REGISTER || HARD_REGNO_OK_FOR_BASE_P(NUM))#endif/* A C expression which is nonzero if register number NUM is suitable for use as an index register in operand addresses. */#define REGNO_OK_FOR_INDEX_P(NUM) 0

The macros REGNO_OK_FOR_BASE_P and REGNO_OK_FOR_INDEX_P have two versions – the normal version that accepts all pseudo registers as well as the valid hard registers, and one “strict” version that only accepts the valid hard registers. The strict version is used from within the register allocator, so it needs to check pseudo registers against the reg_number array to accept cases where the pseudo register is in the process of being allocated to a hard register.

The back end also needs to implement the TARGET_ADDR_SPACE_LEGITIMATE_ADDRESS_P hook that is used by the compiler to check that the address expression is valid (this is needed as the hardware may have more detailed constraints that cannot be expressed by the macros above). This is done similarly to the valid_address_p example in “inspecting the IR” above, but it needs to handle the “strict” case too. The Moxie back end implements this as

Tuesday, August 22, 2017

Compilers are usually described as having three parts – a front end that handles the source language, a middle that optimizes the code using a target-independent representation, and a back end doing the target-specific code generation. GCC is no different in this regard – the front end generates a target-independent representation (GIMPLE) that is used when optimizing the code, and the result is passed to the back end that converts it to its own representation (RTL) and generates the code for the program.

The back end IR

The back end’s internal representation for the code consists of a linked list of objects called insns. An insn corresponds roughly to an assembly instruction, but there are insns representing labels, dispatch tables for switch statements, etc.. Each insnsn is constructed as a tree of expressions, and is usually written in an LISP-like syntax. For example,

(reg:mn)

is an expression representing a register access, and

(plus:mxy)

represents adding the expressions x and y. An insn adding two registers may combine these as

(set (reg:mr0) (plus:m (reg:mr1) (reg:mr2)))

The m in the expressions denotes a machine mode that defines the size and representation of the data object or operation in the expression. There are lots of machine modes, but the most common are

QI – “Quarter-Integer” mode represents a single byte treated as an integer.

HI – “Half-Integer” mode represents a two-byte integer.

SI – “Single Integer” mode represents a four-byte integer.

DI – “Double Integer” mode represents an eight-byte integer.

CC – “Condition Code” mode represents the value of a condition code (used to represent the result of a comparison operation).

GCC supports architectures where a byte is not 8 bits, but this blog series will assume 8-bit bytes (mostly as I often find it clearer to talk about a 32-bit value in examples, instead of a more abstract SImode value).

Overview of the back end operation

The back end runs a number of passes over the IR, and GCC will output the resulting RTL for each pass when -fdump-rtl-all is passed to the compiler.

The back end starts by converting GIMPLE to RTL, and a small GIMPLE function such as

The generated RTL corresponds mostly to real instructions even at this early stage in the back end, but the generated code it is inefficient and registers are still virtual.

The next step is to run optimization passes on the RTL. This is the same kind of optimization passes that have already been run on GIMPLE (constant folding, dead code elimination, simple loop optimizations, etc.), but they can do a better job with knowledge of the target architecture. For example, loop optimizations may transform loops to better take advantage of loop instructions and addressing modes, and dead code elimination may see that the operations working on the upper part of

on a 32-bit architecture are not needed (as the returned value is always 0) and can thus be eliminated.

After this, instructions are combined and spit to better instruction sequences by peep-hole optimizations, registers are allocated, the instructions are scheduled, and the resulting RTL dump contains all the information about which instructions are selected and registers allocated

which defines an insn with a name mulsi3 that generates a mul instruction for a 32-bit multiplication.

The first operand in the instruction pattern is a name that is used in debug dumps and when writing C++ code that generates RTL. For example,

emit_insn (gen_mulsi3 (dst, src1, src2));

generates a mulsi3 insn. The back end does not in general need to generate RTL, but the rest of GCC does, and the name tells GCC that it can use the pattern to accomplish a certain task. It is therefore important that the named patterns implement the functionality that GCC expects, but names starting with * are ignored and thus safe to use for non-standard instruction patterns. The back end should only implement the named patterns that make sense for the architecture – GCC will do its best to emit code for missing patterns using other strategies. For example, a 32-bit multiplication will be generated as a call to __mulsi3 in libgcc if the target does not have a mulsi3 instruction pattern.

The next part of the instruction pattern is the RTL template that describes the semantics of the instruction that is generated by the insn

This says that the instruction multiplies two registers and places the result in a register. This example is taken from RISC-V that has a multiplication instruction without any side-effects, but some architectures (such as X86_64) sets condition flags as part of the operation, and that needs to be expressed in the RTL template, such as

where the parallel expresses that the two operations are done in as a unit.

The instruction’s operands are specified with expressions of the form

(match_operand:SI 1 "register_operand" "r")

consisting of four parts:

match_operand: followed by the machine mode of the operand.

The operand number used as an identifier when referring the operand.

A predicate telling what kind of operands are valid ("register_operand" means that it must be a register).

A constraint string describing the details of the operand ("r" means it must be a general register). These are the same constraints as are used in inline assembly (but the instruction patterns support additional constraints that are not allowed in inline assembly).

The predicate and constraint string contain similar information, but they are used in different ways:

The predicate is used when generating the RTL. As an example, when generating RTL for a GIMPLE function

then the GIMPLE to RTL converter will generate the multiplication as a mulsi3 insn. The predicates will be checked for the result _2 and the operands a_1(D) and 42, and it is determined that 42 is not valid as only registers are allowed. GCC will, therefore, insert a movsi insn that moves the constant into a register.

The constraints are used when doing the register allocation and final instruction selection. Many architectures have constraints on the operands, such as m68k that has 16 registers (a0–a7 and d0–d7), but only d0–d7 are allowed in a muls instruction. This is expressed by a constraint telling the register allocator that it must use register d0–d7.

The string after the RTL template contains C++ code that disables the instruction pattern if it evaluates to false (an empty string is treated as true). This is used when having different versions of the architecture, such as one for small CPUs that do not have multiplication instructions, and one for more advanced cores that can do multiplication. This is typically handled by letting machine.opt generate a global variable TARGET_MUL that can be set by an option such as -mmul and -march, and this global variable is placed in the condition string

so that the instruction pattern it is disabled (and the compiler will thus generate a call to libgcc) when multiplication is not available.

The resulting instruction is emitted using the output template

"mul\t%0,%1,%2"

where %0, %1, ..., are substituted with the corresponding operands.

More about define_insn

Many architectures need more complex instruction handling than the RISC-V mul instruction described above, but define_insn is flexible enough to handle essentially all cases that occur for real CPUs.

Let’s say our target can multiply a register and an immediate integer and that this requires the first operand to be an even-numbered register, while multiplying two registers requires that the first operand is an odd-numbered register (this is not as strange as it may seem – some CPU designs use such tricks to save one bit in the instruction encoding). This is easily handled by defining a new predicate in predicates.md

The constraint strings now list two alternatives – one for the register/register case and one for the register/integer case. And there is a % character added to tell the back end that the operation is commutative (so that the code generation may switch the order of the operands if the integer is the first operand, or help the register allocation for cases such as

_1 = _2 * 42;
_3 = _2 * _4;

to let it change the order of arguments on the second line to avoid inserting an extra move to make _2 an even register on the first line and an odd register on the second line).

Sometimes the different alternatives need to generate different instructions, such as the instruction multiplying two registers being called mulr and the multiplication with an integer being called muli. This can be handled by starting the output template string with an @ character and listing the different alternatives in the same order as in the constraint strings

This is usually not that useful for define_insn but may reduce the number of instruction patterns when the instruction names depend on the configuration. For example, mulsi3 in the RISC-V back end must generate mulw in 64-bit mode and mul in 32-bit mode, which is implemented as